New phone + existing SIM card

I recently bought a Nexus 5 from Google and would like to use my existing Koodo SIM card from my old phone in this new phone. I inserted the SIM card without issue. I am able to make and receive phone calls no problem. Sending/receiving SMS with Hangouts is also no problem. However, I've noticed that I cannot send or receive MMS. I checked APN settings, but they seem to be fine.

I tried to use the Swap Phone feature in Self Serve to notify Koodo that I have a new phone, but when I select "Upgrade" my existing phone and insert my current SIM #, I get the following error message: "Sorry, this equipment (*******[sim number]) is already in use."

I don't understand -- can't I use my existing SIM in my new phone? I only intend to have one phone on my account.

I guess my real issue is not being able to send or receive MMS. Perhaps someone else familiar with Android can chime in. I have Data usage turned off (because I don't have a data plan and prefer to use Wifi only). However, my Koodo plan includes unlimited SMS and MMS. On my old HTC One V I didn't have trouble with this configuration -- the mobile network was off but I could still send and receive MMS. Now I can't seem to do either of those things. Help?

First of all, make sure you have the right APN settings. Search "Koodo APN" in this forum and you'll find more than enough answers. If that doesn't work, there's a free app on Google Play Store call Tweakker, that will do everything for you and fix anything that may be wrong with your mobile data settings.

You NEED to have mobile data turned on for MMS to work, it's not a wi-fi feature. If it's included in your plan, you won't be paying for it.

It's an unlocked phone from previously from Rogers with a Koodo SIM. And yes, that is something that can be changed, perhaps not through the self serve option, but through customer service. As a former rep, we could do that through our software quite simply by changing the IMEI or SN associated with the SIM card.

REASONING: If I lose my phone, and deactivate my SIM, the handset itself can still be used with a different SIM card. Perhaps I'd be able to prevent that by using Android Device Manager, but this can be circumvented. As an added safeguard, having my IMEI or ESN registered to my account, and associated with my SIM, provides extra safeguards to prevent usage after theft or loss. This is important to me for various reasons including employment and data security.